Many control/positioning systems use actuators to position mechanisms, such as the control surfaces of an aircraft. These actuators may be electromechanical, electrohydraulic, etc. A typical aircraft Automatic Flight Control System (AFCS) includes a dual independent stability augmentation system (SAS) to provide automatic fail-operational capability. Dual systems are usually configured to identify a discrepancy between the actuators. If the system is allowed to continue operating in the event of an actuator run-away, the pilot or copilot may take the required corrective action. While such a system is satisfactory in a dual-pilot aircraft, its use in a single pilot aircraft would be unacceptable. It is known to shut down both actuators in a dual-actuator system to remedy a run-away. However, shutting down both actuators in the event of a failure of one is undesirable since it leaves the pilot without that particular loop of autopilot control. Presently, available systems which will shut down the particular run-away actuator require an arbitrator, either a third actuator or an electronic model, to achieve the shut down and as such are quite complex.
Each actuator in a dual system has separate sensors and electronics. Therefore, while each should respond similarly, some discrepancy is to be expected. Typically, the discrepancy is on the order of only a few percent. These small discrepancies are acceptable, since one actuator generally tends to compensate for discrepancies in the other. Therefore, the position of one actuator may lag the position of its companion, but large positional discrepancies serve as nominal indicators of an actuator failure. Consider however the case where the actuators are at 70% of their travel (authority) and one actuator (or its associated electronics) fails and goes hardover to 0%. The other actuator is left with only 30% authority with which to counteract the hardover and therefore will be unable to compensate fully. In certain control axes, such as pitch, such a failure can be very undesirable.